In the electronics industry, solder flux is often applied to a printed circuit board prior to soldering a lead of an electrical component to a corresponding metallized area on the board. The flux serves both as a cleaning and wetting agent to enhance the quality of the solder bond formed between the leads and metallized areas on the board. Typically, the flux is applied by contacting the circuit board with either a liquid wave or a foam of flux, the latter obtained by bubbling a gas through the flux which is made to foam. Alternatively, flux may be applied to the circuit board by brushing or spraying.
Typically, the solder fluxes used within the electronics industry include at least three basic components, a solid (i.e., non-volatile) activator (e.g., an acid), a solid vehicle (e.g., rosin), and a liquid solvent, (e.g., isopropyl alcohol). More recently, solder fluxes have been developed which utilize a water-soluble vehicle, rather than rosin. Such fluxes are referred to as water-soluble fluxes. As compared to rosin-based fluxes, water-soluble fluxes are more chemically active, and generally achieve higher quality solder bonds. Moreover, any flux residues which remain after soldering must be removed from the board by rinsing with water. Notwithstanding the advantages of water-soluble fluxes, a large number of electronics manufacturers still continue to use rosin fluxes because of their unwillingness or their inability to adjust their manufacturing processes or component designs to accommodate water-soluble fluxes.
While rosin-based fluxes dominate the electronics industry, the use of such fluxes is not without problems. Rosin-based fluxes often leave residues on the circuit board after soldering which interfere with testing of the board. For this reason, circuit boards are usually cleaned after soldering. However, rosin residues are difficult to remove, so that harsh industrial detergents or chlorofluorocarbons must be used to clean the circuit board. The disposal of spent industrial detergents is usually expensive while the use of chlorofluorocarbons has been found to be detrimental to the environment.
In an effort to eliminate the need to clean the circuit board after soldering, several manufacturers are presently marketing liquid low-solids fluxes which are so named because they contain small amounts (e.g., 1-5% by weight) of solids (activator and vehicle). Because of the small amount of solids within such fluxes, the amount of residue left on the board is much reduced, as compared to the residues remaining after the use of conventional rosin fluxes. However, even the small level of residues remaining on the circuit board after soldering with low-solids fluxes can be detrimental to the performance and reliability of the circuit board. For example, when low-solids fluxes are applied by conventional methods, such as by a liquid wave or by foaming, brushing or spraying, we have found that the surface insulation resistance of the insulative areas on the board varies widely, often falling below acceptable limits. Reduction in the surface insulation resistance below a particular level may increase the likelihood of electrical short circuits on the board which will likely adversely affect its operation.
Therefore, there is a need for a technique for applying low-solids flux to a circuit board so that the surface insulation resistance of the insulative areas on the board does not decrease below acceptable limits.